Forming large titanium parts

ABSTRACT

Apparatus for superplastically forming large parts from titanium comprising: a furnace ( 10 ) having an interior, the inside surface of the furnace ( 10 ) being contoured and finished so as to form a mold for the part to be superplastically formed; means ( 70 ) for heating the interior of the furnace ( 10 ); and a supply ( 60 ) of an inert gas. The surface of the mold is adapted to receive a substantially unformed titanium blank ( 80 ). The heating means ( 70 ) is adapted to heat the titanium blank ( 80 ) to the required temperature for superplastic forming. The supply ( 60 ) of the inert gas is operable to exert a pressure onto the surface of the titanium blank ( 80 ) furthermost from the surface of the mold such that the inert gas causes the titanium blank ( 80 ) to deform and take up the shape of the mold, thereby forming the required part. The heating means ( 70 ) includes one or more electrical induction coils positioned in the furnace ( 10 ) so as to be on the side of the titanium blank ( 80 ) furthermost from the mold when that titanium blank ( 80 ) is positioned in the furnace ( 10 ) for superplastic forming in the mold, such that the or each induction coil induces a current in the titanium blank ( 80 ) which is heated thereby.

[0001] This invention relates to the forming of large parts fromtitanium. More particularly, it relates to the forming of such partswith external dimensions of the order of meters.

[0002] Superplastic forming of titanium involves heating titanium sheetto a specified temperature (usually in the region of 950 degreescentigrade) and blow forming the heated titanium by using an inert gas,such as argon. This process is usually performed in either an industrialpress or a furnace. However, the finite dimensions of these two itemsimpose restrictions on the size of the parts that can besuperplastically formed within them. Consequently, parts too large to beaccommodated in existing presses or furnaces have to be fabricated fromtwo or more smaller sections that have been superplastically formedindividually. This increases both the time and cost associated with theforming of large parts from titanium. Furthermore, it may lead todimensional inaccuracies in the finished part owing to its fabricationfrom a number of smaller constituent sections: unacceptably largedimensional variations resulting from the aggregate of the normalindividual dimensional variations.

[0003] U.S. Pat. No. 5,209,093 discloses apparatus for superplasticallyforming large cylindrical structures, comprising:

[0004] (i) a generally cylindrical ceramic die having a radiallyinwardly facing surface defining the contours of a structure to beformed from a cylindrically rolled metal sheet positioned radiallyinwardly therefrom;

[0005] (ii) radiant heating means positioned radially inwardly of therolled metal sheet for heating a medial portion thereof to apredetermined temperature at which it achieves a superplastic condition;and

[0006] (iii) means for introducing a pressurised gas to force the medialportion of the rolled metal sheet radially outwardly against the ceramicdie when the medial portion of the rolled metal sheet has been heated toa superplastic condition.

[0007] The ceramic die may be formed of segments that mate along radialplanes, for example 120 degree segments. This allows the die to bedisassembled to enable removal of titanium structures formed thereinwhich cannot be removed in any other way.

[0008] Radiant heating means tend to heat indiscriminately any objectthat lies in the path of radiation emitted from the heating means.Although this will result in the desired heating of the rolled metalsheet if the sheet is in the path of the emitted radiation, it may alsoresult in objects being unnecessarily heated, such as the die or endportions of the apparatus. As a consequence, a greater amount of energywill be needed to operate the radiant heating means than would be thecase if only the rolled metal sheet were heated. A corollary of this isthat, for any given operating power of the heating means, the rolledmetal sheet will take longer to reach the required temperature under theaction of radiant heating means than would be the case if only therolled metal sheet were heated.

[0009] It is an object of this invention to address these disadvantages.

[0010] According to a first aspect of this invention there is provided amethod of superplastically forming large parts from titanium, comprisingthe steps of:

[0011] a) constructing a furnace, the inside surface of the furnacebeing contoured and finished so as to form a mould for the part to besuperplastically formed;

[0012] b) introducing a substantially unformed titanium blank into thefurnace;

[0013] c) heating the titanium blank to the required temperature forsuperplastic forming;

[0014] d) applying inert gas to the surface of the titanium blankfurthermost from the surface of the mould, wherein the gas pressure issuch that it causes the titanium blank to deform and take up the shapeof the mould, thereby forming the required part, characterised in thatthe heating is by one or more electrical induction coils positioned onthe side of the titanium blank furthermost from the mould such that theor each induction coil induces a current in the titanium blank which isheated thereby.

[0015] The substantially unformed blank may be substantially cylindricaland the or each induction coil may be positioned inside saidsubstantially cylindrical blank.

[0016] The or each induction coil may be orientated such that its axisis substantially parallel to the axis of said substantially cylindricalblank.

[0017] There may be a plurality of such induction coils, the inductioncoils being distributed so that collectively they define an annulus thatis substantially coaxial with the substantially cylindrical blank.

[0018] According to another aspect of this invention there is providedapparatus for superplastically forming large parts from titaniumcomprising: a furnace having an interior, the inside surface of thefurnace being contoured and finished so as to form a mould for the partto be superplastically formed; means for heating the interior of thefurnace; and a supply of an inert gas, wherein the surface of the mouldis adapted to receive a substantially unformed titanium blank, theheating means is adapted to heat the titanium blank to the requiredtemperature for superplastic forming when that titanium blank ispositioned in the furnace for superplastic forming in the mould, and thesupply of the inert gas is operable to exert a pressure onto the surfaceof the titanium blank furthermost from the surface of the mould whenthat titanium blank is positioned in the furnace for superplasticforming in the mould such that the inert gas causes the titanium blankto deform and take up the shape of the mould, thereby forming therequired part, characterised in that the heating means include one ormore electrical induction coils positioned in the furnace so as to be onthe side of the titanium blank furthermost from the mould when thattitanium blank is positioned in the furnace for superplastic forming inthe mould, such that the or each induction coil induces a current in thetitanium blank which is heated thereby.

[0019] The or each induction coil may extend the full height of thesurface of the mould.

[0020] The substantially unformed blank may be substantially cylindricaland the or each induction coil may be positioned inside saidsubstantially cylindrical blank when that titanium blank is positionedin the furnace for superplastic forming in the mould.

[0021] The or each induction coil may be orientated such that its axisis substantially parallel to the axis of said substantially cylindricalblank when that titanium blank is positioned in the furnace forsuperplastic forming in the mould.

[0022] There may be a plurality of such induction coils, the inductioncoils being distributed so that collectively they define an annulus thatis substantially coaxial with the substantially cylindrical blank whenthat titanium blank is positioned in the furnace for superplasticforming in the mould.

[0023] The contoured inside of the furnace surface may be formed of aceramic material. The contoured inside of the furnace surface may beformed of a metallic or nonmetallic material that may be magnetiseableor non-magnetiseable.

[0024] The furnace may be constructed of a plurality of furnace walls orfurnace wall sections.

[0025] The inert gas may be argon.

[0026] To assist the deformation of the titanium blank under the actionof the inert gas, a vacuum may be applied to the side of the blank thatis adjacent the mould.

[0027] The titanium blank may be partially formed, before it isintroduced into the furnace.

[0028] Specific embodiments of the present invention are now describedby way of example and with reference to the accompanying drawings, ofwhich:

[0029]FIG. 1 is a transverse cross-section of a furnace for forming alarge diameter titanium tube by superplastic forming;

[0030]FIG. 2 is an enlarged view of the detail within the circlelabelled A of FIG. 1;

[0031]FIG. 3 is a plan view of the apparatus of FIG. 1;

[0032]FIG. 4 is a plan view of another furnace, wall sections of thefurnace being in mutual abutment; and

[0033]FIG. 5 is another plan view of the other furnace, the wallsections being spaced apart.

[0034]FIGS. 1 and 3 show a cylindrical furnace 10 which is constructedpredominantly from three furnace wall sections 30, each moulded from thesame ceramic material. Collectively, the three sections 30 form thecurved surfaces of a cylinder whose longitudinal axis lies vertically.

[0035] The inside surface of the cylinder formed by the three ceramicwall sections 30 is shaped and finished so as to form a mould suitablefor forming titanium.

[0036] The furnace 10 rests on a base plate 40, which in turn issituated on legs 50. The centre of the base plate 40 includes a lowerargon gas inlet aperture 60 which is connected to a controllable supplyof argon gas.

[0037]FIG. 1 also shows a heating assembly 70. This assembly consists ofa circular top plate 71 with a central top aperture 72 and severalinduction coils. The induction coils are attached to the underside ofthe top plate 71 and are orientated so as to lie vertically and arecircumferentially spaced from one another so as to form a circular arraythat is coaxial with the circular top plate 71. The induction coilsthemselves are not shown, but each of them has a support member 73, ofwhich two are shown in FIG. 1. An annular bottom plate 74 is attached tothe lower end of the support members 73 and is also coaxial with the topplate 71.

[0038] The heating assembly 70 may be inserted into and withdrawn fromthe cylindrical furnace 10. FIG. 1 shows the heating assembly 70inserted into the furnace 10. In this position, the top plate 71 of theheating assembly 70 rests on and is coaxial with the cylinder formed bythe three ceramic wall sections 30. The induction coils extend the fullheight of the inside surface of the sections 30.

[0039] The method of operation is described now with reference toFIG. 1. A substantially unformed titanium blank 80 (in the form of acylinder fabricated from a length of titanium sheet) is placed aroundthe annulus defined by the induction coils. The positioning of the blank80 relative to the heating assembly 70 is such that the top edge of theblank 80 contacts an O-ring seal situated in the top plate 71 of theheating assembly 70 and the bottom edge of the blank 80 contacts anO-ring seal situated in the bottom plate 74 of the heating assembly 70.FIG. 2 shows the O-ring seal 90 included in the top plate 71. Thisprovides airtight contact between this plate 71 and the blank 80. Asimilar arrangement is included in the bottom plate 74. Although FIG. 2shows the top edge of the blank 80 in contact with the top plate 71, itis envisaged that, when the blank is at ambient temperature, a gapshould exist between the top edge of the blank 80 and the top plate 71to allow for thermal expansion of the blank 80.

[0040] The heating assembly 70 and the blank 80 are then inserted intothe centre of the furnace 10. To facilitate this insertion, two of theceramic wall sections 30 are pivotably mounted, thereby serving as doorsto the furnace 10. This arrangement is shown in FIG. 3, the pivotablymounted sections being labelled 30 a, 30 b.

[0041] Closing the two doors 30 a, 30 b causes the top and bottom edgesof the inside surface of all three ceramic sections 30 to abut the topand bottom edges respectively of the titanium blank 80. The inclusion ofseals 90 in the top and bottom edge of the inside surfaces of thesections 30 provides for an airtight contact against the blank 80. Thiscompletes the arrangement shown in FIG. 1. The titanium blank 80 isfirmly held in position by the airtight abutment of the three circularsections 30 on its outside surface and the airtight abutment of the topplate 71 and bottom plate 74 on its inside surface.

[0042] Argon gas is introduced into the furnace 10 through the loweraperture 60 in the base plate 40. The argon gas replaces air that waspreviously inside the furnace by forcing that air out of the topaperture 72. The top aperture 72 is then closed by any known means, suchas a bung or a cut-off valve, and the introduction of argon gas isceased.

[0043] The electrical induction coils are then operated. A current flowsin each coil and this results in a respective associated magnetic fieldbeing set up around the coil. The current in each coil is in the samedirection, thus causing each respective field to be orientated in thesame direction. As a result, a substantially toroidal magnetic field isset up around the annular arrangement of the coils. Magnetic flux ofthis field passes, in an axial direction, through the titanium blank 80that is adjacent and surrounds the annular arrangement of the coils,thereby causing a current to be induced in the titanium blank 80. Thisinduced current results in the titanium blank 80 being heated.Positioning the annular arrangement of coils inside the titanium blank80 does not optimise the induction heating effect of the coils as far asheating the titanium blank 80 is concerned. This is because the fluxdensity outside the annular arrangement of coils is less than the fluxdensity inside the annular arrangement of coils. Positioning the annulararrangement of coils inside the titanium blank 80 therefore puts thetitanium blank 80 in a weaker part of the field. However, by positioningthe annular arrangement of coils inside the titanium blank 80, it ispossible to more accurately predict how the titanium blank will beheated, as each of the coils is at a known and easily verifiabledistance from the surface of the blank 80. Furthermore, the coils may bemore easily replaced in the event of failure, or altered in order toachieve different heating characteristics. One such alteration may be tomove some of the turns of one or more coils apart and others of theturns of the or each coil together, in order to achieve a differentheating profile of the titanium blank 80.

[0044] Using induction is advantageous as compared with using radiantheating means. Induction coils may be used to avoid heating parts of theapparatus that need not be heated, for example the circular top plate 71or the base plate 40, if such parts are fabricated fromnon-magnetiseable material. The use of induction coils therefore resultsin improved efficiency and a shorter heating time for any givenoperating power of the induction coils. A shorter heating time isadvantageous in reducing the thermal stress to which components of theapparatus are subjected. This may result in prolonging the useful lifeof the components, or in the use of cheaper components. For exampleconventional O-ring seals may be used to provide a gas-tight seal whilstpermitting movement of the blank due to thermal expansion. It will beappreciated that high temperature, mechanical-type seals may hinder suchexpansion and increase the likelihood of the blank 80 buckling.

[0045] Once the titanium blank 80 has been heated to the requiredtemperature for superplastic forming, more argon gas is introduced intothe furnace via the aperture 60. This is continued such that thepressure of the argon on the inside surface of the titanium blank 80 isgreater than the pressure against the outside surface of the blank 80,the two spaces being sealed from one another in an airtight fashion aspreviously described. This pressure difference causes the titanium blank80 to deform outwards and take up the shape of the mould comprised ofthe inside surface of the three ceramic sections 30. To further increasethe pressure difference across the two surfaces of the titanium blank80, it is envisaged that a vacuum may be applied to the outside surfaceof the titanium blank 80. Techniques for achieving this are known to theskilled addressee.

[0046] The heating is then stopped, allowing the superplastically formedpart to cool prior to removal from the furnace 10 and the heatingapparatus 70.

[0047] In an alternative embodiment, shown in FIG. 4, the furnace wallsections 30 are not pivotably mounted. Instead, the furnace wallsections 30 are surrounded by a cylindrical outer wall 100. Thecylindrical outer wall 100 is coaxial with the wall sections 30 and hasan internal diameter that is greater than the external diameter of thewall sections 30. Thus, when the wall sections 30 are in mutualabutment, there is an annular space 110 between the wall sections 30 andthe cylindrical outer wall 100. Six actuators 120, only three of whichare shown, are mounted on the outer surface of the outer cylindricalwall 100. A pair of actuators 120 are provided for each wall section 30:an upper actuator 120 and a lower actuator 120.

[0048] Each pair of actuators 120 are positioned so that their lines ofaction pass radially through a respective one of the wall sections. Eachactuator includes an actuator rod 125. The actuator rods 125 of eachpair of actuators 120 pass through the cylindrical outer wall 100 andmechanically engage the respective wall section 30. Operation of theactuators 120 causes the wall sections 30 to be moved radially between aposition in which they are in mutual abutment and a position in whichthey are spaced apart. FIG. 4 shows the wall sections 30 in mutualabutment. It will be appreciated that the heating and forming operationswould be performed in this position. FIG. 5 shows the wall sections 30spaced apart. It will be appreciated that it is in this position thatthe titanium blank would be inserted into the furnace 10, the heatingassembly 70 (not shown) would be inserted into and withdrawn from thefurnace 10, and the formed titanium part (not shown) would be withdrawnfrom the furnace 10.

1. A method of superplastically forming large parts from titanium,comprising the steps of: a) constructing a furnace (10) with an insidesurface which is contoured and finished so as to form a mould for thepart to be superplastically formed; b) introducing a substantiallyunformed titanium blank (80) into the furnace (10); c) heating thetitanium blank (80) to the required temperature for superplasticforming; d) applying inert gas to the surface of the titanium blank (80)furthermost from the mould, wherein the gas pressure is such that itcauses the titanium blank (80) to deform and take up the shape of themould, thereby forming the required part, characterised in that theheating is by one or more electrical induction coils positioned on theside of the titanium blank (80) furthermost from the mould such that theor each induction coil induces a current in the titanium blank (80)which is heated thereby.
 2. A method according to claim 1, wherein theor each induction coil extends the full height of the mould.
 3. A methodaccording to claim 1 or claim 2, wherein the substantially unformedblank (80) is substantially cylindrical and the or each induction coilis positioned inside said substantially cylindrical blank (80).
 4. Amethod according to claim 3, wherein the or each induction coil isorientated such that its axis is substantially parallel to the axis ofsaid substantially cylindrical blank (80).
 5. A method according toclaim 3 or claim 4, wherein there is a plurality of such inductioncoils, the induction coils being distributed so that collectively theydefine an annulus that is substantially coaxial with the substantiallycylindrical blank (80).
 6. A method according to any one of thepreceding claims, wherein the furnace (10) is constructed from aplurality of furnace walls or furnace wall sections (30) whichcollectively form the inside surface of the furnace (10).
 7. A methodaccording to claim 6, wherein the or each furnace wall or furnace wallsection (30) is formed of a ceramic material.
 8. A method according toany one of the preceding claims, wherein the inert gas is argon. 9.Apparatus for superplastically forming large parts from titaniumcomprising: a furnace (10) having an interior, the inside surface of thefurnace (10) being contoured and finished so as to form a mould for thepart to be superplastically formed; means (70) for heating the interiorof the furnace (10); and a supply (60) of an inert gas, wherein thesurface of the mould is adapted to receive a substantially unformedtitanium blank (80), the heating means (70) is adapted to heat thetitanium blank (80) to the required temperature for superplastic formingwhen that titanium blank (80) is positioned in the furnace (10) forsuperplastic forming in the mould, and the supply (60) of the inert gasis operable to exert a pressure onto the surface of the titanium blank(80) furthermost from the surface of the mould when that titanium blank(80) is positioned in the furnace (10) for superplastic forming in themould such that the inert gas causes the titanium blank (80) to deformand take up the shape of the mould, thereby forming the required part,characterised in that the heating means (70) include one or moreelectrical induction coils positioned in the furnace (10) so as to be onthe side of the titanium blank (80) furthermost from the mould when thattitanium blank (80) is positioned in the furnace (10) for superplasticforming in the mould, such that the or each induction coil induces acurrent in the titanium blank (80) which is heated thereby. 10.Apparatus according to claim 9, wherein the or each induction coilextends the full height of the mould.
 11. Apparatus according to claim 9or claim 10, wherein the substantially unformed blank (80) issubstantially cylindrical and the or each induction coil is positionedin the furnace (10) so as to be inside said substantially cylindricalblank (80) when that titanium blank (80) is positioned in the furnace(10) for superplastic forming in the mould.
 12. Apparatus according toclaim 11 wherein the or each induction coil is orientated such that itsaxis is substantially parallel to the axis of said substantiallycylindrical blank (80) when that titanium blank (80) is positioned inthe furnace (10) for superplastic forming in the mould.
 13. Apparatusaccording to claim 11 or claim 12, wherein there is a plurality of suchinduction coils, the induction coils being distributed so thatcollectively they define an annulus that is substantially coaxial withthe substantially cylindrical blank (80) when that titanium blank (80)is positioned in the furnace (10) for superplastic forming in the mould.14. Apparatus according to any one of claims 9 to 13, wherein thefurnace (10) is constructed of a plurality of furnace walls or furnacewall sections (30), which collectively form the inside surface of thefurnace (10).
 15. Apparatus according to claim 14, wherein the or eachfurnace wall or furnace wall section (30) is formed of a ceramicmaterial.
 16. Apparatus according to any one of claims 9 to 15, whereinthe inert gas is argon.